Biaxially oriented pipe with a thickened end portion

ABSTRACT

In an aspect, a biaxially oriented pipe has a thickened end portion. The end portion has the same inner diameter as the biaxially oriented pipe and has a larger thickness than the biaxially oriented pipe. The end portion is made of the same thermoplastic polymer composition as the biaxially oriented pipe.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is divisional application of US16/498,837 filed Sep.27, 2019 which claims priority to PCT/EP2018/057705 filed Mar. 27, 2018that claims the benefit of European Application No. 17163917.2, filedMar. 30, 2017. The related applications are incorporated herein byreference in their entirety.

BACKGROUND

The present invention relates to a modification process for modifying abiaxially oriented pipe. The present invention further relates to a pipejoining process of the so-obtained modified pipes.

It is known to improve the physical and mechanical properties of apolymer material by orienting the material. In many cases, orienting amaterial to improve a property in one direction leads to worsening ofthe same property in the direction perpendicular to the direction oforientation. In order to adapt the properties in both directions, abiaxial orientation of the material may be applied. The biaxialorientation means that the polymer material is oriented in twodirections, perpendicular to one another. A pipe can be oriented in theaxial direction and peripheral direction (hoop direction) to improveproperties such as tensile strength.

End portions of pipes made of a biaxially oriented polymer can be joinedby methods such as butt welding. However, when the butt ends are meltedfor butt welding, the pipe will have a lower burst pressure than beforethe butt welding since the orientation is lost at the butt ends.Accordingly, biaxially oriented polymer pipes are normally joined bymechanical methods in order to preserve their orientation. Another knownway for joining pipes is by using electrofusion techniques as describedby Atkinson et.al., POLYM ENG SCI., 1989, Vol.29, No.23, p.1638-1641. Itis desirable to provide a pipe joining process which avoids a largedecrease of the mechanical properties at the joint.

It is an object of the invention to provide a process in which theabove-mentioned and/or other problems are solved.

BRIEF SUMMARY

The invention relates to a modification process for modifying abiaxially oriented pipe, comprising a) providing a biaxially orientedpipe made by stretching a tube made of a thermoplastic polymercomposition in the axial direction and in the peripheral direction, b)placing an insert within an end portion of the pipe, wherein the outerperiphery of the cross section of the insert substantially matches theinner periphery of the cross section of the pipe and c) heating the endportion such that the end portion axially shrinks while the innerperiphery of the cross section of the end portion is substantiallymaintained, to obtain a modified biaxially oriented pipe with athickened end portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a heating tool used for preparation of theend portion of a pipe at time t=0. FIG. 2 is an illustration of theheating tool used for preparation of the end portion of the pipe at timet =10 minutes.

DETAILED DESCRIPTION

Accordingly, the invention provides a modification process for modifyinga biaxially oriented pipe, comprising

a) providing a biaxially oriented pipe made by stretching a tube made ofa thermoplastic polymer composition in the axial direction and in theperipheral direction,

b) placing an insert within an end portion of the pipe, wherein theouter periphery of the cross section of the insert substantially matchesthe inner periphery of the cross section of the pipe and

c) heating the end portion such that the end portion axially shrinkswhile the inner periphery of the cross section of the end portion issubstantially maintained, to obtain a modified biaxially oriented pipewith a thickened end portion.

The invention further provides a pipe joining process, comprising

I) performing the modification process according to the invention toobtain a first modified biaxially oriented pipe and a second modifiedbiaxially oriented pipe and

II) joining the butt-end of the thickened end portion of the firstbiaxially oriented pipe and the butt-end of the thickened end portion ofthe second biaxially oriented pipe.

The terms “pipe” and “tube” are herein understood as a hollow elongatedarticle. The cross section may be of various shapes e.g. be circular,elliptical, square, rectangular or triangular. The term “diameter” isherein understood as the largest dimension of the cross section.

The outer periphery of the cross section of the insert substantiallymatches the inner periphery of the cross section of the pipe. This meansthat the outer periphery of the cross section of the insert and theinner periphery of the cross section of the pipe have the same shape(i.e. both of them are circular or both of them are square etc.) and thediameter of the outer periphery of the cross section of the insert isequal to or slightly smaller than the diameter of the inner periphery ofthe cross section of the pipe, e.g. the diameter of the outer peripheryof the cross section of the insert is 95-100% of the diameter of theinner periphery of the cross section of the pipe.

According to the modification process of the invention, a matchinginsert is placed within one end of the biaxially oriented pipe. This endportion of the pipe is heated close to the melting temperature of thepipe material while the rest of the pipe is not heated. Upon heating,the degree of orientation of the thermoplastic polymer at the endportion is decreased and the end portion shrinks axially while its innerperiphery is maintained due to the presence of the insert. This resultsin the end portion with an increased thickness.

These thickened end portions can be joined at their butt-ends withoutany further change in their dimensions. The joining results in an almostcomplete loss of the orientation at the butt-end, as in the conventionalprocesses. However, according to the modification process of theinvention, a relatively high burst pressure at the joined butt ends isobtained. While the loss of orientation at the joined butt ends reducesthe burst pressure, the increased thickness of the end portion increasesthe burst pressure. Consequently, the burst pressure at the joined buttends is much higher than the joined butt ends made according toconventional processes.

It was found that during the formation of the thickened end portion thedegree of orientation in the thickened end portion is retained to alarger degree in the hoop direction than in the axial direction.Further, during the joining step the orientation is retained to acertain degree in the thickened end portion except for the butt end. Theretention of the orientation in the joined end portions may furthercontribute to the maintenance of the burst pressure.

The preparation of the biaxially oriented pipe, modifying the endportion of the pipe and joining the so-obtained pipes can advantageouslybe performed in the same plant site. Alternatively, modifying and thejoining of the pipes can be performed in the pipe laying site. Thepossibility to perform the modification and the joining of the pipes atthe production site or the pipe laying site is highly advantageous forthe efficiency of the pipe installation. This is made possible by thefact that no special equipment is required, unlike conventionalmechanical joining methods.

Step a)

In step a), a biaxially oriented pipe made by stretching a tube made ofa thermoplastic polymer composition in the axial direction and in theperipheral direction is provided. Preferably, the stretching has beenperformed in the axial direction at an axial draw ratio of 1.1 to 5.0and in the peripheral direction at an average hoop draw ratio of 1.1 to2.0.

Preferably, Step a) Involves

a1) forming a thermoplastic polymer composition into a tube and

a2) stretching the tube of step a1) in the axial direction at an axialdraw ratio of 1.1 to 5.0 and in the peripheral direction at an averagehoop draw ratio of 1.1 to 2.0 to obtain the biaxially oriented pipe.

Step a1) Thermoplastic Polymer Composition

Preferably, thermoplastic polymer composition comprises a thermoplasticpolymer selected from the group consisting of polyethylene,polypropylene, polyvinylchloride, polyester, polycarbonate, polyamide,polyacetal, polyimide, polyvinylidene fluoride and polyether etherketone and combinations thereof.

A preferred example of the thermoplastic polymer is polyethylene, suchas high-density polyethylene (HDPE), linear low-density polyethylene(LLDPE) and low-density polyethylene (LDPE), and particularly preferredis high-density polyethylene (HDPE). Another preferred example of thethermoplastic polymer is polypropylene, preferably random polypropylene.

Polyethylene

The production processes of HDPE, LLDPE and LDPE are summarised inHandbook of Polyethylene by Andrew Peacock (2000; Dekker; ISBN0824795466) at pages 43-66.

HDPE

HDPE may be an ethylene homopolymer or may comprise a comonomer, forexample butene or hexene.

Preferably, the HDPE has a density of 940-960 kg/m³ ,more preferably940-955 kg/m³ , measured according to ISO1183.

Preferably, the HDPE has a Melt Flow Rate of 0.1-4 g/10 min, morepreferably 0.1-1 g/10min, measured according to ISO1133-1:2011 (190 °C/5 kg).

In some embodiments, the composition comprises a compound comprising theHDPE and a colorant, wherein the compound has a density of 947-965 kg/m³measured according to ISO1183. The colorant may e.g. be carbon black ora pigment having a color of e.g. black, blue or orange. The amount ofthe colorant is typically 1-5 wt.%, more typically 2-2.5 wt.%, withrespect to the compound comprising the HDPE and the colorant, the resttypically being the HDPE.

The HDPE may be unimodal, bimodal or multimodal. Preferably, the HDPE isbimodal or multimodal. Such HDPEs have properties suitable for producinga pipe.

It is understood that a bimodal HDPE has a molecular weight distributionhaving two peaks corresponding to the first median and the second medianof the respective stages in the polymerization. It is similarlyunderstood that a multimodal HDPE has a molecular weight distributionhaving multiple peaks corresponding to the first median, the secondmedian and one or more further medians of the respective stages in thepolymerization.

The HDPE can be produced by using low pressure polymerisation processes.For example, pipe materials of the performance class PE 80 and PE 100are known, which are generally produced in cascade plants by a so calledbimodal or multimodal process. The production processes for bimodal HDPEare summarised at pages 16-20 of “PE 100 Pipe systems” (edited byBromstrup; second edition, ISBN 3-8027-2728-2).

Suitable low pressure processes are slurry cascade of stirred reactors,slurry cascade of loop reactors and a combination of different processessuch as slurry loop gas phase reactor. It is also possible to use amultimodal polyethylene, preferably trimodal polyethylene, as describedfor example in WO2007003530, as high density polyethylene pipe material.

The performance classes PE 80 and PE 100 are discussed at pages 35- 42of “PE 100 Pipe systems” (edited by Bromstrup; second edition, ISBN3-8027-2728- 2). The quality test methods are described at pages 51-62of “PE 100 Pipe systems”.

The production of bimodal high density polyethylene (HDPE) via a lowpressure slurry process is described by Alt et al. in “Bimodalpolyethylene-Interplay of catalyst and process” (Macromol.Symp. 2001,163, 135-143). In a two stage cascade process the reactors may be fedcontinuously with a mixture of monomers, hydrogen, catalyst/co-catalystand hexane recycled from the process. In the reactors, polymerisation ofethylene occurs as an exothermic reaction at pressures in the rangebetween for example 0.5 MPa (5 bar) and 1 MPa (10 bar) and attemperatures in the range between for example 75 ° C. and 85 ° C. Theheat from the polymerisation reaction is removed by means of coolingwater. The characteristics of the polyethylene are determined amongstothers by the catalyst system and by the applied concentrations ofcatalyst, co monomer and hydrogen.

The concept of the two stage cascade process is elucidated at pages137-138 by Alt et al. “Bimodal polyethylene-Interplay of catalyst andprocess” (Macromol.Symp. 2001, 163). The reactors are set up in cascadewith different conditions in each reactor including low hydrogen contentin the second reactor. This allows for the production of HDPE with abimodal molecular mass distribution and defined co monomer content inthe polyethylene chains.

Preferred examples of the HDPE include a unimodal or bimodal PE 80, abimodal PE 100 and a multimodal HDPE resin. PE 80 is a PE material withan MRS (minimum required strength after 50 years for water at 20 degreesCelsius) of 8 MPa and PE 100 is a PE material with an MRS of 10 MPa. Thepipe classification is elucidated at page 35 of “PE 100 Pipe systems”(edited by Bromstrup; second edition, ISBN 3-8027-2728- 2).

Preferably, the HDPE or the compound comprising the HDPE and thecolorant has one or more of, preferably all of, the followingcharacteristics:

- Tensile modulus of 500-1400 MPa, preferably 700-1200 MPa (according toISO 527-2)

- Yield stress of 15-32 MPa, preferably 18-28 MPa (according to ISO527-2)

- Full Notch Creep Test (FNCT): 100 - 20000 h (according to ISO 16770 @80 degrees centigrade / 4 MPa)

- Charpy of 10-35 ºC @ 23 ºC., preferably 14-30 º(according to ISO 1eA).

LLDPE

The technologies suitable for the LLDPE manufacture include gas-phasefluidized-bed polymerization, polymerization in solution, polymerizationin a polymer melt under very high ethylene pressure, and slurrypolymerization.

The LLDPE comprises ethylene and a C3-C10 alpha-olefin comonomer(ethylene-alpha olefin copolymer). Suitable alpha-olefin comonomersinclude 1-butene, 1-hexene, 4-methyl pentene and 1-octene. The preferredco monomer is 1-hexene. Preferably, the alpha-olefin co monomer ispresent in an amount of about 5 to about 20 percent by weight of theethylene-alpha olefin copolymer, more preferably an amount of from about7 to about 15 percent by weight of the ethylene-alpha olefin copolymer.

Preferably, the LLDPE has a density of 900-948 kg/m³, more preferably915-935 kg/m³, more preferably 920-935 kg/m³, determined according toISO1872-2.

Preferably, the LLDPE has a Melt Flow Rate of 0.1-3.0 g/10min, morepreferably 0.3-3.0 g/10min, determined according to ISO1133-1:2011 (190°C/2.16kg).

LDPE

The LDPE may be produced by use of autoclave high pressure technologyand by tubular reactor technology.

LDPE may be an ethylene homopolymer or may comprise a comonomer, forexample butene or hexene.

Preferably, the LDPE has a density of 916-940 kg/m³, more preferably920-935 kg/m³, determined according to ISO1872-2.

Preferably, the LLDPE has a Melt Flow Rate of 0.1-3.0 g/10min, morepreferably 0.3-3.0 g/10min, determined according to ISO1133-1:2011 (190°C/2.16kg).

Polyethylene Composition

Preferably, the thermoplastic polymer composition comprises HDPE. Insome embodiments, the polyethylene composition comprises a furtherpolyethylene other than HDPE. The further polyethylene may e.g. belinear low-density polyethylene (LLDPE), low-density polyethylene (LDPE)or a combination of LLDPE and LDPE. Preferably, the further polyethyleneis LLDPE or a combination of LLDPE and LDPE.

More preferably, the further polyethylene is LLDPE. In case the furtherpolyethylene is a combination of LLDPE and LDPE, the weight ratio ofLLDPE to LDPE may e.g. be at least 0.1, for example at least 0.2 or atleast 0.3 and at most 10, for example at most 5 or at most 3.Preferably, the weight ratio of LLDPE to LDPE is at least 1, for example2 to 10. Preferably, the weight ratio of HDPE to the furtherpolyethylene in the polyethylene composition is more than 1, preferably1.2-5, for example 1.5-4 or 2-3.

In some embodiments, the thermoplastic polymer composition essentiallycomprises no further polyethylene other than HDPE. The amount of HDPE inpolyethylene in the polyethylene composition may be at least 95 wt%, atleast 98 wt.%, at least 99 wt.% or 100 wt.%.

Preferably, the thermoplastic polymer composition comprising HDPE has aMelt Flow Rate of 0.1-4 g/10 min, more preferably 0.1-1 g/10min,measured according to ISO1133-1:2011 (190 ºC/5 kg).

Other Polymers

Another preferred example of the thermoplastic polymer is polypropylene,preferably a random propylene copolymer.

The term “random propylene copolymer,” as used herein, is a copolymercontaining monomers of propylene and monomers of α-olefin polymerizedtogether to form a polymer wherein the individual repeating units arepresent in a random or statistical distribution in the polymer chain.

Additives

The thermoplastic polymer composition may comprise components other thanthe thermoplastic polymer, such as additives and fillers.

Examples of the additives include nucleating agents; stabilisers, e.g.heat stabilisers, anti-oxidants, UV stabilizers; colorants, likepigments and dyes; clarifiers; surface tension modifiers; lubricants;flame-retardants; mould-release agents; flow improving agents;plasticizers; anti-static agents; external elastomeric impact modifiers;blowing agents; and/or components that enhance interfacial bondingbetween polymer and filler, such as a maleated polyethylene. The amountof the additives is typically 0 to 5 wt.%, for example 1 to 3 wt.%, withrespect to the total composition.

Examples of fillers include glass fibers, talc, mica, nanoclay. Theamount of fillers is typically 0 to 40 wt.%, for example 5 to 30 wt.% or10 to 25 wt.%, with respect to the total composition.

Accordingly, in some embodiments, the composition further comprises 0 to5 wt.% of additives and 0 to 40 wt.% of fillers.

The thermoplastic composition may be obtained by melt-mixing thethermoplastic polymer optionally with any other optional components.

Preferably, the total amount of the thermoplastic polymer and theoptional additives and the optional fillers is 100 wt.% with respect tothe total thermoplastic polymer composition.

In some embodiments, the total amount of polyethylene with respect tothe total amount of polymers present in the thermoplastic polymercomposition is at least 95 wt.%, at least 98 wt.%, at least 99 wt.% or100 wt.%.

In some embodiments, the total amount of polyethylene with respect tothe total thermoplastic polymer composition is at least 90 wt%, at least95 wt.%, at least 98 wt.%, at least 99 wt.% or 100 wt.%.

Process Steps The thermoplastic polymer composition may be formed into atube (step a1) by any known method, such as extrusion or injectionmoulding. The biaxial elongation (step a2) may be performed by any knownmethod.

Methods for forming a thermoplastic polymer composition into a tube andthe biaxial elongation of the tube are described in US6325959:

A conventional plant for extrusion of plastic pipes comprises anextruder, a nozzle, a calibrating device, cooling equipment, a pullingdevice, and a device for cutting or for coiling-up the pipe. By themolten mass of polymer on its way from the extruder through the nozzleand up to calibration, cooling and finished pipe being subjected toshear and elongation etc. in the axial direction of the pipe, anessentially uniaxial orientation of the pipe in its axial direction willbe obtained. A further reason that contributes to the orientation of thepolymer material in the direction of material flow is that the pipe canbe subjected to tension in connection with the manufacture.

To achieve biaxial orientation, this plant can be supplemented,downstream of the pulling device, with a device for temperature controlof the pipe to a temperature that is suitable for biaxial orientation ofthe pipe, an orienting device, a calibrating device, a cooling device,and a pulling device which supplies the biaxially oriented pipe to acutting device or coiler.

The biaxial orientation can also be carried out in direct connectionwith the first calibration after extrusion, in which case theabove-described supplementary equipment succeeds the first calibratingdevice.

The biaxial orientation of the pipe can be carried out in various ways,for instance mechanically by means of an internal mandrel, or by aninternal pressurised fluid, such as air or water or the like. A furthermethod is the orienting of the pipe by means of rollers, for instance byarranging the pipe on a mandrel and rotating the mandrel and the piperelative to one or more pressure rollers engaging the pipe, or viainternally arranged pressure rollers that are rotated relative to thepipe against an externally arranged mould or calibrating device.

Conditions for Step a2)

Preferably, step a2) is performed at a drawing temperature which is 1 to30 ºC lower than the melting point of the thermoplastic polymercomposition, for example 2 to 20 ºC or 3 to 10 ºC lower than the meltingpoint of the thermoplastic polymer composition. When more than onemelting point can be measured for the thermoplastic polymer composition,step b) is preferably performed at a drawing temperature which is 1 to30 ºC lower than the highest melting point of the thermoplastic polymercomposition, for example 2 to 20 ºC or 3 to 10 ºC lower than the highestmelting point of the thermoplastic polymer composition.

In embodiments where the thermoplastic polymer comprises HDPE, step a2)may also be performed at a drawing temperature which is 1 to 30 ºC lowerthan the melting point of the HDPE, for example 2 to 20 ºC or 3 to 10 ºClower than the melting point of the HDPE.

In some embodiments, step a2) is performed at a drawing temperature of115-123 ºC.

Preferably, the axial draw ratio is 1.1 to 5. Preferably, the axial drawratio is at least 1.2, at least 1.3, at least 1.5 or at least 1.8 and/orat most 4.0, at most 3.5, at most 3.2, at most 3.0, at most 2.8 or atmost 2.5. Preferably, the average hoop draw ratio is at least 1.2 or atleast 1.3 and/or at most 1.8 or at most 1.6.

The axial draw ratio of the drawn pipe is defined as the ratio of thecross-sectional area of the starting isotropic tube to that of thebiaxially oriented pipe (i.e. product), that is,

$\lambda_{axial} = \frac{\left( {{Tube}{OD}} \right)^{2} - \left( {{Tube}{ID}} \right)^{2}}{\left( {{Product}{OD}} \right)^{2} - \left( {{Product}{ID}} \right)^{2}}$

OD stands for outer diameter and ID stands for inner diameter.

The average hoop draw ratio can be defined as:

$\lambda_{{average}{hoop}} = \frac{{Total}{Draw}{Ratio}\lambda_{Total}}{{Axial}{Draw}{Ratio}\lambda_{axial}}$${{Where}\lambda_{Total}} = \frac{{Tube}{Wall}{Thickness}}{{Product}{Wall}{Thickness}}$

Biaxially Oriented Pipe

The biaxially oriented pipe of step a) may be a pressure pipe or anon-pressure pipe.

The preferred pipe is a pressure pipe.

The biaxially oriented pipe may typically have a thickness of 0.3 mm to100 mm. The biaxially oriented pipe may typically have an outer diameterof 2 mm to 2000 mm. In some examples, the biaxially oriented pipe has anouter diameter of 2 mm to 10 mm and a thickness of 0.3 mm to 2 mm. Insome examples, the biaxially oriented pipe has an outer diameter of 10mm to 100 mm and a thickness of 1 mm to 3 mm. In some examples, thebiaxially oriented pipe has an outer diameter of 100 mm to 500 mm and athickness of 1 mm to 10 mm. In some examples, the biaxially orientedpipe has an outer diameter of 500 mm to 2000 mm and a thickness of 5 mmto 100 mm.

In some examples, the biaxially oriented pipe has an outer diameter of32 mm to 110 mm and a thickness of 3 mm to 10 mm. Examples of suitablebiaxially oriented pipes of step a) have the following outer diameterand inner diameter and wall thickness.

Outer diameter (mm) Inner diameter (mm) Wall thickness (mm) 110 90 10.090 73.6 8.2 75 61.4 6.8 63 51.4 5.8 32 26 3.0

Step b) and Step c)

In step b), an insert is placed within an end portion of the pipe.

In step c), the end portion is heated such that the end portion axiallyshrinks while the inner periphery of the cross section of the endportion is substantially maintained. A modified biaxially oriented pipewith a thickened end portion is thereby obtained.

Step c) may involve heating the end portion from the inside and/or theoutside of the end portion. It was found that heating from the inside ofthe end portion reduces the degree of axial and peripheral orientationat the inner surface, but the degree of peripheral orientation islargely maintained at the outer surface. Similarly, heating from theoutside of the end portion reduces the degree of axial and peripheralorientation at the outer surface, but the degree of peripheralorientation is largely maintained at the inner surface.

When a high production speed is important, the end portion is preferablyheated from the inside and the outside of the end portion.

The location of the heating influences where the orientation ismaintained. Heating only from the inside or only from the outside of theend portion can be used to influence the degree of orientation. Whenmaintaining the degree of orientation is important, the end portion ispreferably heated only from the outside of the end portion. The degreeof orientation in the peripheral direction is typically higher at theinner surface than the outer surface. Accordingly, the reduction in thedegree of orientation affects the inner surface more than the outersurface. By avoiding heating from the inside of the end portion, theperipheral orientation at the inner surface is maintained.

When the end portion is heated at least from the inside of the endportion, step c) preferably involves heating at least part of theinsert.

In some embodiments, the insert comprises a thermally conductive portionand a thermally insulating portion. In this case, step b) involvesplacing the insert in the pipe such that the thermally conductiveportion is closer to the butt-end of the end portion of the pipe thanthe thermally insulating portion. This may typically be done by placingthe insert such that the end of the thermally conductive portion isflush with the butt-end of the end portion of the pipe. Step c) involvesheating only the thermally conductive portion of the insert. Thethermally insulating portion may suitably be made of a metal, e.g.steel. The thermally insulating portion may e.g. be made of nylon.

Preferably, in step c), the thermally conductive portion of the endportion is heated at a temperature at or higher than the drawingtemperature. The heating temperature in step c) may e.g. be up to 5 ºC.higher than the drawing temperature.

Preferably, the thickened end portion has a thickness which is 110-250%of the original thickness of the end portion.

Preferably, the thickened end portion has a minimum ultimate tensileload of at least 80%, preferably at least 90%, more preferably at least100%, of the minimum ultimate tensile load of the end portion of theoriginal pipe measured by ASTM D2290.

Modified Pipe The invention also relates to the modified biaxiallyoriented pipe obtainable or obtained by the modifying process accordingto the invention. Pipe Joining Process

The invention provides a pipe joining process. In step I), themodification process according to the invention is performed to obtain afirst modified biaxially oriented pipe and a second modified biaxiallyoriented pipe. In step II), the butt-end of the thickened end portion ofthe first biaxially oriented pipe and the butt-end of the thickened endportion of the second biaxially oriented pipe are joined. This mayinvolve butt-welding, solvent joining or electrofusion.

Butt-welding involves heating the end portions of the pipes to above themelting point of the pipe material and attaching the butt ends, followedby cooling.

In solvent welding, a solvent is applied which can temporarily dissolvethe polymer at room temperature. The dissolved butt ends are attached.Given sufficient time, the solvent will permeate through the polymer andout into the environment, so that the chains lose their mobility.

Electrofusion is described in Atkinson et.al., POLYM ENG SCI., 1989,Vol.29, No.23, p.1638-1641.

Joined Pipe

The invention also relates to the joined pipe obtainable or obtained bythe pipe joining process according to the invention.

The invention also relates to a joined pipe comprising two biaxiallyoriented pipes and a joint portion between the two biaxially orientedpipes, wherein the biaxially oriented pipes has an outer diameter of 2mm to 2000 mm and a thickness of 0.3 mm to 100 mm and are obtained bydrawing a tube made of a thermoplastic polymer composition at an axialdraw ratio of 1.1-5.0 and a hoop draw ratio of 1.1-2.0, wherein thejoint portion is made of the same thermoplastic polymer composition asthe tube,

wherein the two biaxially oriented pipes and the joint portion have thesame inner diameter and

the joint portion has a larger thickness than the biaxially orientedpipes.

Preferably, the joint portion has a thickness which is 110-250% of thethickness of the biaxially oriented pipes.

Preferably, the joint portion has a minimum ultimate tensile load of atleast 80%, preferably at least 90%, more preferably at least 100%, ofthe minimum ultimate tensile load of the biaxially oriented pipesmeasured according to ASTM D2290.

Use

The invention also relates to use of the joined pipe according to theinvention for pressure pipes for gas, water and industrial applicationsor building and constructions applications such as scaffolding and roofsupport.

It is noted that the invention relates to all possible combinations offeatures described herein, preferred in particular are thosecombinations of features that are present in the claims. It willtherefore be appreciated that all combinations of features relating tothe composition according to the invention; all combinations of featuresrelating to the process according to the invention and all combinationsof features relating to the composition according to the invention andfeatures relating to the process according to the invention aredescribed herein.

It is further noted that the term ‘comprising’ does not exclude thepresence of other elements. However, it is also to be understood that adescription on a product/composition comprising certain components alsodiscloses a product/composition consisting of these components. Theproduct/composition consisting of these components may be advantageousin that it offers a simpler, more economical process for the preparationof the product/composition. Similarly, it is also to be understood thata description on a process comprising certain steps also discloses aprocess consisting of these steps. The process consisting of these stepsmay be advantageous in that it offers a simpler, more economicalprocess.

When values are mentioned for a lower limit and an upper limit for aparameter, ranges made by the combinations of the values of the lowerlimit and the values of the upper limit are also understood to bedisclosed.

The invention is now elucidated by way of the following examples,without however being limited thereto.

Examples Preparation Of Die-Drawn Pipes

Circular HDPE tubes of outer diameter 60 mm and inner diameter 25 mmwere melt extruded. These thick isotropic tubes were drawn over aconical mandrel of exit diameter 59 mm at a temperature of 120 ºC.

Biaxially oriented pipes were produced using the batch die-drawingfacility. A series of biaxially oriented pipes with an outer diameter of64.9(±0.8)mm and an inner diameter of 56.6(±0.3)mm were prepared bydrawing at an axial draw ratio of about 3.5 and an average hoop drawratio of about 1.4.

Preparation Of Thickened End Portions

A cylindrical insert of the same outer diameter as the inner diameter ofthe oriented pipe was placed inside one end of the biaxially orientedpipe. This insert is made of a cylinder made of steel and a cylindermade of nylon joined together. The insert was inserted in the orientedpipe such that the end of the steel cylinder is flush with the butt-end.The steel cylinder was heated to 130 - 140ºC., close to the meltingtemperature of the HDPE pipe material while the rest of the pipe in itsvicinity was kept well below this temperature over the nylon cylinder.This setup is shown in FIGS. 1 and 2. FIGS. 1 and 2 illustrate theheating tool used for preparation of the end portions of pipes at timet=0 and t=10 minutes, respectively.

Upon heating the pipe end, the pipe end started shrinking axially whilethe inner diameter was maintained due to constraint by the insert. Theouter and inner diameters of the end portions changed from values of64.9(±0.8)mm and 56.6(±0.3)mm to 70.6(±0.4)mm and 54.5(±0.5)mm. Theprocess ultimately resulted in a significant increase in the wallthickness of the end portion from 4.1mm to 8mm.

Burst Pressures

The burst pressures of pipes can be calculated via Lame relation

$P = {S\frac{\left( {D^{2} - d^{2}} \right)}{\left( {D^{2} + d^{2}} \right)}}$

where P represents the burst pressure, S the minimum ultimate tensilestrength of pipe material, D the outer pipe diameter and d the innerpipe diameter. The minimum ultimate tensile strength of pipe material ismeasured as hoop tensile strength of pipes measured using split diskmethod ASTM D2290. Table 1 shows the variations in burst pressures,calculated for different values of the pipe dimensions to illustrate theinfluence of the pipe wall thickness.

TABLE 1 S D d Wall thickness P pipe [MPa] [mm] [mm] [mm] [MPA] isotropic20   63 51.4 5.8 4.0 biaxially 35   63 51.4 5.8 7.0 oriented annealedend 25¹⁾ 70 51.4 9.3 7.5 25¹⁾ 72 51.4 10.3 8.1 20²⁾ 70 51.4 9.3 6.0 20²⁾72 51.4 10.3 6.5 ¹⁾Partial loss of orientation ²⁾Complete loss oforientation at the jointsThese calculations demonstrate that the joints having a high wallthickness have a high burst pressure, which will compensate for the lossof the orientation.

Changes In The Crystalline Morphology Related To Heating

A series of pole figure diagrams were derived from WAXD experiments oneach sample, machined from different positions within the pipe crosssections.

It was found that the heating resulted in melting and a loss oforientation within a very narrow region near the inner pipe surface. Theorientation at the outer pipe surface was maintained to a large degree.There was a higher decrease in the degree of orientation in the axialdirection than in the hoop direction.

Brief Description of the Drawing Numbers

100 Heat

110 Oriented Pipe

120 Thermally Insulating Cylinder

130 Heater

140 Metal Cylinder

150 Axial Shrinkage Claims

1. A biaxially oriented pipe with a thickened end portion, wherein thethickened end portion has the same inner diameter as the biaxiallyoriented pipe and has a larger thickness than the biaxially orientedpipe, and wherein the thickened end portion is made of a samethermoplastic polymer composition as the biaxially oriented pipe.
 2. Thebiaxially oriented pipe according to claim 1, wherein the biaxiallyoriented pipe was obtained by drawing a tube made of the thermoplasticpolymer composition at an axial draw ratio of 1.1 to 5.0 and a hoop drawratio of 1.1 to 2.0.
 3. The biaxially oriented pipe according to claim1, wherein the thermoplastic polymer comprises polyethylene,polypropylene, polyvinylchloride, polyester, polycarbonate, polyamide,polyacetal, polyimide, polyvinylidene fluoride, polyether ether ketone,or a combination thereof.
 4. The biaxially oriented pipe according toclaim 1, wherein the thermoplastic polymer comprises high densitypolyethylene or random polypropylene.
 5. The biaxially oriented pipeaccording to claim 1, wherein the thickened end portion has a thicknesswhich is 110 to 250% of a thickness of the biaxially oriented pipe. 6.The biaxially oriented pipe according to claim 1, wherein a thickness ofthe biaxially oriented pipe is 0.3 mm to 100 mm.
 7. The biaxiallyoriented pipe according to claim 1, wherein an outer diameter of thebiaxially oriented pipe is 2 mm to 2000 mm.
 8. The biaxially orientedpipe according to claim 1, wherein the thickened end portion has aminimum ultimate tensile load of at least 80% of the minimum ultimatetensile load of the biaxially oriented pipe measured according to ASTMD2290.
 9. The biaxially oriented pipe according to claim 1, wherein thethickened end portion has a minimum ultimate tensile load of at least100% of the minimum ultimate tensile load of the biaxially oriented pipemeasured according to ASTM D2290.
 10. The biaxially oriented pipeaccording to claim 1, wherein the thickened end portion has a minimumultimate tensile load of at least 90% of the minimum ultimate tensileload of the biaxially oriented pipe measured according to ASTM D2290.11. A joined pipe obtained by joining a butt-end of the thickened endportion of the biaxially oriented pipe of claim 1 with a butt-end of asecond thickened end portion of a second biaxially oriented pipe.
 12. Ajoined pipe comprising two biaxially oriented pipes and a joint portionbetween the two biaxially oriented pipes, wherein the two biaxiallyoriented pipes each have an outer diameter of 2 mm to 2000 mm and athickness of 0.3 mm to 100 mm and are obtained by drawing a respectivetube made of a thermoplastic polymer composition at an axial draw ratioof 1.1 to 5.0 and a hoop draw ratio of 1.1 to 2.0, wherein the jointportion is made of the same thermoplastic polymer composition as the twobiaxially oriented pipes, wherein the two biaxially oriented pipes andthe joint portion have the same inner diameter and the joint portion hasa larger thickness than the biaxially oriented pipes.